docs: def _get_site_amplification_non_linear(self, vs30, pga4nl, C):
          """
          Compute site amplification non-linear term,
          equations (8a) to (13d), pag 108-109.
          """
          # non linear slope
          bnl = self._compute_non_linear_slope(vs30, C)
          # compute the actual non-linear term
          return self._compute_non_linear_term(pga4nl, bnl)